Multiple modules, often assembled in a cassette, are deployed in a tank of fluid to be filtered, e.g. water, or wastewater with suspended solids, either for batch filtration, batch-continuous filtration, or less likely, continuous filtration, all processes which are described in the prior art. In all instances, filtration is accompanied with air injected (“sparging air”) into the tank so as to generate bubbles which scrub the fibers of a skein to remove solids left on the surfaces of its fibers. Without such scrubbing action, suspended solids in the wastewater would build up on the surfaces of the fibers and around them, and greatly impede filtration.
It is self-evident from the foregoing that, irrespective of the filtration process used, the greater the spacing between fibers, that is, the more open a skein, and the longer the air bubbles can contact the fibers, the longer will their surfaces remain clean, and the better will be the performance of the module. However, the more open a skein, the lower is the surface area of fibers per unit volume of substrate in which the module is deployed, and the higher the cost of using the module.
The prior art has devoted much effort towards configuring a module for a commercially viable process, but has provided little indication as to which features in a module coax the maximum efficiency from a chosen skein of fibers. Modules in which the skein is subdivided into bundles of fibers potted at one end in an upper header and at their opposed ends into multiple lower headers are disclosed in US 2010/0237014, (hereafter “the '014 application”, or “'014” for brevity). Improving filtration efficiency is the goal of modules deployed as disclosed in U.S. Pat. No. 6,899,811. The configuration of the novel module disclosed herebelow provides enhanced filtration efficiency.
Packing Density of Fibers in a Header:
The prior art has decreed that an economical skein should have as high a packing density “δ” of fibers in a header as is practical with fibers of a given nominal (outer) diameter from 1-3 mm, chosen fiber-to-fiber spacing, and supply of fiber-cleansing air; the length of the fibers is chosen so as to derive maximum benefit from upwardly streaming air bubbles, while avoiding fiber damage because of fiber-to-fiber interaction or fiber shrinkage. Therefore fibers of the aforesaid diameter, in a header, whether upper or lower, in a prior art elongated rectangular module, typically have a maximum practical packing density “δh” of about 0.32 (32%), referred to as “max header density”.
The term “packing density” of fibers in a header, sub-divided header or miniheader refers to [the total area occupied by the circular cross-sections of all fibers (based on the outer diameter “d” of each) clustered or bundled within the boundaries in the x-y plane, and within the vertical x-z and y-z planar boundaries of the header, or sub-divided header, or miniheader (referred to as “header”), in which the terminal portions of fibers are secured, whether in arrays or randomly clustered], divided by, [the planar area of the header's footprint in the x-y plane]. A limited number of arrays, that is, from 2-12 arrays, preferably 2-8 arrays, most preferably 2-6 arrays, planarly clustered sequentially, adhesively and back-to-back, the number of fibers in each array limited to 250, are referred to as a miniskein. By “back-to-back” is meant that a pair of arrays may be secured adhesively with a mounting strip therebetween, or with adhesive only, all other arrays, if present, being secured with adhesive only.
For example, a cluster of “n” fibers having an outer diameter “d” occupy a total area of [n=πd2/4] cm2. When the n fibers are anchored in a rectangular header having length “x” cm (measured along the x-axis) and width “y” cm (measured along the y-axis), the footprint of the header is “xy” cm2. The packing density of fibers δh secured in the miniheader is [(n·πd2/4)/xy].
The use of a planar ordered array (or “array” for brevity) to form a header is disclosed in U.S. Pat. No. 5,639,373 and U.S. Pat. No. RE39,294 wherein terminal ends of fibers are fixed, side-by-side on only one side of the mounting strip which provides a physical boundary or barrier for the array. A second such array is prepared on another (second) mounting strip and overlaid upon the first; then a third ordered array is similarly prepared on a third mounting strip and overlaid on the second; and the procedure is further repeated to form a stack of arrays on one side of the mounting strip. The stack of multiple planar ordered arrays (or “arrays” for brevity) on mounting strips is then collectively potted to function as a header.
With a view to optimizing operating costs, a first embodiment illustrated in the '014 application, teaches a bundle of fibers (a “skein”) in an elongated rectangular upper header x cm long and y cm wide, arranged in a number of generally parallel sheets or planes (referred to hereinafter as “arrays”); and, the bundle is divided longitudinally into four (4) sub-bundles which are potted in 4 lower sub-headers each ‘xs’ cm long and ‘ys’ cm wide, spaced-apart by ‘w’ cm, the grouped sub-bundles forming a composite open lower header. The term “composite” refers to the header comprising plural sub-bundles, each of which is formed by sub-dividing the skein of the module. The term “open” refers to the spaces between adjacent sub-divided headers which spaces provide through-passage for air bubbles rising between each sub-divided header to scrub the fibers of the sub-bundle.
In another embodiment is shown a module (“400”) in which each sub-bundle is an elongated rectangular cluster of arrays in each of three sub-bundles forming the module (see FIG. 11 of the '014 application). In the first embodiment, the 4 spaced-apart sub-bundled lower headers provide open spaces, each xw cm2 in area, therebetween. Each sub-bundle provides a sub-header having an area xs ys cm2. The grouped lower sub-headers are therefore referred to herein as a “composite open lower header”.
The area occupied by “n” fibers potted in each of four (4) sub-bundles is 4xs ys cm2 which is less than the total area [4xs ys+3xs w] cm2 occupied by the composite open lower header (which is the footprint of the composite open lower header). If the fibers were sub-bundled and spaced apart in the upper header as shown in FIG. 2, the upper header would be analogously referred to as a “composite upper header” but not a composite open upper header, because the rectangular spaces between spaced-apart sub-bundles in the upper header are filled with potting material and cannot provide an open through passage.
The packing density of the sub-bundled fibers in each sub-header is [(n·πd2/4)/xs ys]/cm2. The “effective packing density” (δhe) of fibers in each sub-header refers to a ratio computed as {the total area 4[n·πd2/4] cm2 of all the fibers} divided by {the planar area of the footprint, whether xy cm2 of a header, or xcyc cm2 of a composite header having length “xc” and width “yc”}, over which footprint the fibers are distributed. The δhe of fibers in the composite lower header is [4(n·πd2/4)/(4xs ys+3xs w)].
If “n” identical fibers having outer diameter “d” are used in a skein the upper ends of which are potted in an upper header A and the lower ends potted in a lower header B, each with the same footprint xy, then δhe of the fibers in each header is the same, irrespective of how the fibers are distributed in each header.
If the n identical fibers are used in a skein the upper ends of which are potted in an upper header A having area xA yA cm2 and the lower ends in a lower header, or composite header, B having a different area xB yB cm2, the δhe for A and B will be different.
It is self-evident that when the area of the lower header is greater than that of the upper header, the δhe for lower header B will be lower than that of the upper header A irrespective of whether the length or width of lower header is greater than those of upper header A, or both length and width are greater. In either case the skein will be more open near the lower header than near the upper header because the δhe for lower header B will be lower than that for the upper header A. Such openness is expected to provide better contact with rising air bubbles and more readily discharged solids which collect at the base of the skein. What cannot be known or knowledgeably calculated is what configuration of skein will provide a combination of a high enough practical packing density δh with a low enough effective packing density δhe to yield an unexpectedly effective and economically viable module.
Splitting a Skein:
To obtain more effective use of air in a header with desirable packing density, the '373 patent and U.S. Pat. No. 5,910,250 disclose splitting a skein by longitudinally splitting the rectangular upper and lower headers (see FIG. 9 of '373) with a defined open channel for an air-supply tube, with additional air-supply tubes on either side of the bottom header, if desired; and in FIG. 10 of '373 is illustrated two individual skeins which are laterally spaced apart, and cleansing or scrubbing air is introduced between the lower headers.
The foregoing concept of splitting a skein is used in the module disclosed in the '014 application in which module the skein is separated into plural, sub-bundled and separated, longitudinal lower headers (“sub-headers” herein), each having a large number of fibers the lower ends of which are dead-ended (plugged) and potted.
The foregoing principles were embodied in the disclosure of the '373 patent which specified that the terminal portions of potted fibers be spaced apart with center-to-center distance of adjacent fibers preferably in the range from about 1.2-5 times (1.2 d-5 d) the outside diameter ‘d’ of a fiber, with the admonition that spacing the fibers closer increases the risk of fiber-to-fiber contact near the terminal end portions when the ends are potted (see top of col 17).
There is no disclosure in '014 relating to the packing density in each sub-bundle, or the spacing between sub-bundles, so neither the packing density nor the effective packing density of fibers in each header is known. Neither is there any data in the '014 specification indicating that the sub-bundled skein is more cost-effective than a '373 module with conventional, unitary skein, that is, one in which the fibers are not sub-bundled, or one that is split longitudinally as shown in FIG. 9 of the '373 patent.
Shrinkage and Damage:
It is a characteristic of fibers made from synthetic resinous materials (polymers as opposed to ceramics) that they shrink while operating in liquids as they age (not just by aging but by certain specific conditions of temperature and pressure), when the module is put into service. The amount of shrinking depends upon the type of polymer, the diameter “d” of the fiber, and the type of reinforcement, if any, in the fiber. Because the vertical distance “v” (measured along the z-axis) between the upper and lower headers in a typical currently used module is fixed, if the fibers are only slightly, that is in the range from 0.5%-2% longer than “v”, shrinkage during operation over time, tends to break them. If the fibers are substantially longer, that is in the range from 5-10% longer than “v”, the fibers sway in the aerated substrate, usually wastewater, due to the force of uprising gas bubbles, causing a scrubbing action between adjacent fibers. The longer the fibers, the more they sway, and the greater the likely damage. Damaged fibers leak. A single broken and protruding fiber in an operating skein may be sufficient to have the skein removed from service.
As a result, experience in the field with skeins of commonly used polymeric fibers, and the prior art, suggests that, for headers spaced at a vertical distance ‘v’, fibers are used which are in the range from 0.5%˜5% longer than ‘v’. At the upper end of the range, the slack in the fibers may cause damage due to rubbing against each other, and at the lower end of the range, risk breakage due to being overstretched.
In the '014 disclosure, the fibers are potted in various arrangements. For example, fibers in a sub-bundle may be randomly arranged in a lower header, or potted as a stack of mounting strips, each with an array, as disclosed in '373. The upper ends of fibers of each of the multiple sub-bundles (four shown in '014) may be mixed in the upper header; or, the upper ends of fibers of one sub-bundle may be kept from mixing with fibers of other adjacent sub-bundles in the upper header. In the case where the sub-bundling is preserved in the upper header, the spacing between the fibers may be increased, and the spacing between adjacent sub-bundles decreased relative to the spacing in the lower headers. In the case of fibers arranged in arrays, the rows may be generally evenly spaced in the headers, but at a greater spacing in the upper header (see [0042] of the '014 application).
In other words, the '014 disclosure teaches that there is nothing critical about the arrangement of the fibers either in the upper header or in the subdivided lower headers; and no significance is accorded to the packing density δh within a subdivided header, and/or, the effective packing density δhe in the header.
As an illustrative example, an '014 upper header may have from 8 to 30 arrays and be from 5 to 20 cm in width, the length of an array (number of fibers in an array) being indeterminate; a lower header may have from 1 to 5 arrays and be from 0.5 to 4 cm in width, the length of an array being indeterminate. The headers may be elongated in plan view having a ratio of length to width of, for example, 2 or more or 4 or more or 8 or more.
Volumetric Packing Density of Fibers in a Module in Service in a Tank:
One cannot predict performance of a skein from its volumetric packing density. In particular, there is no indication in '014 as to the length of the fibers, or as to the footprint of the headers, or the vertical distance between the upper and lower headers; or, whether the area of the lower header may be greater than that of the upper header; therefore the volumetric packing density of the '014 skein fibers in the wastewater is not known. The term “volumetric packing density” (“δy”) refers to the ratio of [the volume of all ‘n’ fibers each having a cross-sectional area “πd2/4” and length “L” cm between upper and lower headers vertically spaced apart at ‘v’ cm, each header having a footprint xy cm2] to [the volume of wastewater occupied by the skein of fibers]. The ratio is computed as δv=[n πd2/4·L]/[xy·v]. The more fibers of given length and given fiber spacing ‘s’, that can be packed into a fixed volume of substrate, the higher the volumetric packing density. When the fibers are relatively straight and taut between upper and lower modules, that is, v=L, then δv will be equal to the packing density “δh”.
With fibers of length v≈L, and appropriate fiber spacing ‘s’, higher δv nevertheless allows longer and better contact, therefore better scrubbing with uprising air bubbles because the fibers are more vertical and straighter. When the fibers are much longer than the vertical distance between headers, they will sway back and forth in an uprising stream of air bubbles in an irregularly shaped wastewater column larger than the rectangular parallelpiped xyv, the boundaries of which larger irregular column are constantly changing.
In a comparison of two modules using the same fibers, with the vertical distance ‘v’ between the opposed faces being the same, and the areas of the upper and lower headers being the same, and the fiber spacing in each header being the same, δv would be expected to be the same. Thus the effect of δv on performance of each module would be expected to be the same.
To improve the performance one might consider decreasing δv to allow more space between fibers, not increasing δv, which would provide opportunities for fibers to scrub against each other, and increases the chance of one or more fibers being damaged. Yet, by sub-bundling the fibers in headers of the '014 composite lower header, and laterally spacing the sub-bundled headers apart, it is clear δv would be increased in each sub-bundle, and also in the adjacent lower portions of the volume of substrate.
In a comparison of two modules, if each module is used for filtering the same wastewater with the same stated amount of air per unit fiber surface, in the same way, that is, cycling the air in a particular manner, any difference in performance of the modules will be attributable to: (i) the difference in packing densities (Δδh) of fibers in each header or sub-divided header; (ii) the difference in effective packing densities (Δδhe) of fibers in each header, or aerator box (or “box” for brevity); (iii) the difference in volumetric packing densities (Δδy); and, (iv) the difference in effective volumetric packing densities (Δδve) of the same number of fibers in a header having an area larger or smaller than the one for which δv was calculated.
In a comparison of the performance of two modules, a difference in performance must be attributable to differences in the foregoing properties which differences are determined by the packing densities of fiber in each, whether and how the headers are sub-divided, and how the sub-divided headers are configured in each header.
The term “performance” refers to the flux or rate of flow of permeate through a unit of membrane surface (liters/m2/hr) (LMH) or (gals/ft2/day) for each module at a specified rate of flow of air (liters/min/m2) or (m3/m2/hr) or (scfm/ft2) of filtration surface, while filtering wastewater which has a fouling rate of 0.1 kPa/min.
The Problem:
The prior art has failed to appreciate the interaction (i) of packing density of fibers in a header of arrays (the term “array” is used herein to denote “a planar ordered array” unless stated otherwise), (ii) of effective packing density of fibers in headers, (iii) of volumetric packing density of fibers in the column of wastewater occupied by the fibers of a skein in operation, and (iv) of effective volumetric packing density of fibers in the column of wastewater occupied by the fibers distributed over a larger area in one header compared to the area in the other, as they together factor into the performance of a module.
Performance of most fiber filtration modules having the same filtration area are expected to be about the same. However, much like in a marathon, it is the long term performance, that is the module's performance after several days of continuous operation under normally encountered filtration conditions, or over a period of at least 48 hours under accelerated sludge-forming conditions, that makes the difference. The prior art also failed to realize that there is an upper limit to the number of arrays overlappingly secured to a single mounting strip because efficient use of filtration surface requires that no more than 73.5%, preferably no more than 52% of the fibers in a lower header be surrounded (that is, fully encircled) by other adjacent fibers.
The goal is to design and construct a more efficient, reliably operable, hence more economical, shell-less module than any available in the prior art.